1. Field of the Invention
The present invention relates to a method for producing hematite for ironmaking, in which high-purity hematite, which can be used as an iron-making raw material in place of iron ore, is produced by a wet process.
2. Description of the Related Art
In steel smelting, a method of charging iron ore containing iron oxide into a blast furnace along with a reductant such as coke, heating and melting the iron ore under a reducing atmosphere to obtain crude steel, and refining the crude steel in a converter to obtain desired steel has been used.
The iron oxide that is a raw material of the steel is a limited resource, and furthermore it is gradually hard to obtain high-quality iron ore required to maintain a quality of steel.
Meanwhile, with respect to nickel becoming a raw material of stainless steel, technology for smelting low-grade oxide ore as a raw material due to a tendency toward resource exhaustion of sulfide ore that has been used in the past has been developed and put to practical use.
To be specific, nickel oxide ore such as limonite or saprolite is put into a pressure device such as an autoclave along with a sulfuric acid, and nickel is leached under high pressure and high temperature of about 240 to 260° C.
The nickel leached into a solution of the sulfuric acid is used as nickel metal or a nickel salt compound by adding a neutralizer to neutralize a surplus acid, separating a leach residue by solid-liquid separation, separating impurities to recover the leach residue as an intermediate raw material in the form of hydroxide or sulfide, and further refining the intermediate raw material.
In such a process called high pressure acid leach (HPAL), nickel can be almost completely leached even from low-grade ore in which valuable metals intended for recovery are contained by not more than 1% to 2%. Further, the HPAL process has a feature of concentrating the valuable metals up to the same grade as a conventional raw material by producing an intermediate raw material from a leachate, and refining the nickel in a process similar to a conventional process.
Further, the HPAL process may be applied to various types of ores such as nickel sulfide ore, copper sulfide ore, and copper oxide ore, in addition to the nickel oxide ore.
Further, a main component of the leach residue obtained by the HPAL process is iron oxide having the form of hematite. This is secondarily obtained because each of oxide ore and sulfide ore of nickel or copper used as a raw material contains iron of an amount far more than a content of nickel or copper.
These leach residues are created at a high temperature, and thus have the form of oxide that is chemically or environmentally stable. However, the leach residues have no special utility value, and have been scrapped to a residue disposal yard. For this reason, it has been a grave challenge how to secure the disposal yards for an enormous amount of leach residues generated along with the smelting.
Furthermore, the leach residue of the HPAL process cannot be used for the aforementioned iron-making raw material. The reason is that the leach residue of the HPAL process also contains gangue and impurities in addition to the iron oxide, and thus is not suitable for the iron-making raw material.
Particularly, calcium is not preferred as the iron-making raw material, and generally needs to be suppressed to a level of 0.1% or less. For example, in the case of the nickel oxide ore, the calcium is hardly contained in the ore, but as described above, the calcium is derived from quicklime or limestone added to neutralize the surplus acid contained in a leach slurry, and is precipitated in the form of calcium sulfate along with the neutralization.
Therefore, an attempt to use a neutralizer such as magnesium hydroxide or magnesium oxide which contains no calcium has been made. However, in comparison with the calcium neutralizers, the magnesium hydroxide has good reactivity, but is expensive to lead to supply instability, and is unfit for industrial massive use. Additionally, a neutralizer such as sodium hydroxide is too expensive to be practical industrially.
Thus, to increase supply stability and to achieve lower costs, magnesium contained in the ore itself has been considered to be used as the neutralizer.
For example, JP 2009-520661 A discloses a method of recovering magnesium oxide from a source of magnesium sulfate, which includes a process of preparing a source of solution-state magnesium sulfate obtained from a part of a process associated with leaching of metal-containing ore or concentrate, a process of converting the solution-state magnesium sulfate into solid magnesium sulfate, a process of bringing the solid magnesium sulfate into contact with elemental sulfur under a reducing atmosphere, and a process of recovering magnesium as magnesium oxide gas and sulfur as sulfur dioxide gas.
With the use of this method, the magnesium contained in the ore can be reused as the neutralizer, and suppress the introduced calcium, so that it is possible to reduce the calcium mixed into the iron oxide in the residue.
However, the method of JP 2009-520661 A requires a large quantity of heat to crystallize the magnesium in the solution as the magnesium sulfate or to heat the obtained magnesium sulfate into the magnesium oxide, and is far from an economical method.
In contrast, there is a proposal for a method of using, as a neutralizer, a host rock (also called a bedrock or a foundation rock) which is not typically a target to be used as a resource and which is simultaneously mined from a site where nickel oxide ore is mined.
The host rock has a composition shown in, for instance, Table 1, and has a feature of being rich in magnesium. The magnesium contained in the host rock is mainly magnesium oxide, and can also be used as a neutralizer.
TABLE 1NiFeCoSiMgCrAlHost0.224.92<0.0217.422.10.260.13RockMnCaSHost0.090.08<0.05Rock
For example, JP 4294685 B1 discloses a method of recovering nickel or cobalt from oxide ore containing nickel or cobalt and iron, which method includes a process of preparing, as the oxide ore, first oxide ore and second oxide ore having a higher magnesium content than the first oxide ore; a classification process of classifying the first oxide ore into first small particle size oxide ore and first large particle size oxide ore and classifying the second oxide ore into second small particle size oxide ore and second large particle size oxide ore; a process of leaching the nickel or the cobalt from the first large particle size oxide ore using a sulfuric acid and obtaining a sulfuric acid leachate containing the nickel or the cobalt and a leach residue; a reacting process of mixing the sulfuric acid leachate containing the leach residue and the second large particle size oxide ore, reacting the sulfuric acid leachate with the magnesium contained in the second large particle size oxide ore to adjust pH, and obtaining a reaction solution containing the nickel or the cobalt and a reaction residue containing the iron; and a neutralization process of neutralizing the reaction solution containing the reaction residue using a neutralizer and obtaining a neutralization solution containing the nickel or the cobalt and a neutralized residue containing the iron.
With the use of this method, the nickel oxide ore itself can be used as the neutralizer.
However, costs and labor for classifying the ore are unable to be disregarded. In addition, many gangue components are also present in the leach residue, and an iron grade is low as it is, so that the nickel oxide ore is far from an efficient raw material.
The invention is intended to provide a method for producing hematite for ironmaking, capable of recovering high-purity hematite, which can be used as an iron-making raw material, from a leach residue containing iron oxide produced by a high pressure acid leach (HPAL) process in an inexpensive and efficient way.